When it comes to writing big checks to favored candidates and causes, unions last year seemed to be taking greater advantage of the landmark Citizens United decision than corporations.

A Sunlight analysis of groups and individuals who wrote checks of $10,000 or more to super PACs and other political committees that report to the FEC revealed big labor bested big business in 2013 by better than 2-to-1.

Our study was focused on determining who is writing the kind of checks that would not have been legal prior to the controversial 2010 Supreme Court decision that opened the way for unions and companies to give money directly from their treasuries in unlimited amounts — as opposed to the donations they traditionally made in $1,000 increments from their long-established political action committees.

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A series of court cases now known collectively as “Citizens United” paved the way for super PACs and “hybrid” super PACs to accept unlimited donations from groups like corporations and unions that previously were forbidden from giving directly to campaigns. For years, corporations and unions have maintained their own PACs, paid for by contributions from employees or union members; this analysis looked only at money given directly from the treasuries of corporations or unions, as best we could determine.

The lone deep-pocketed donor remains the most important source of big political checks: Of about $108 million raised by super PACs and hybrid super PACs in 2013 in chunks of $10,000 or more, at least $55 million came from individuals, $19 million came from unions, $7 million came from corporations, $3 million came from nonprofits and $1 million came from trusts. Another $16 million came from PACs and other explicitly political organizations (like the Democratic Governor’s Association, a “527” organizations).

It’s important to remember that donations reported to the FEC are just a fraction of the amount of money being raised — much of which is now done in secret. It’s possible that corporations gave far greater sums to political nonprofits that do not have to report their donors during the same time period. What the numbers make clear, as USA Today reported, is that unions are becoming increasingly willing to flex their muscles in an off-year.

Though most donations from corporations went to right-leaning groups, the year’s biggest corporate donor, at $1.1 million, was the Mostyn Law Firm, run by Democratic donor Steve Mostyn. Contran Corporation, founded by the late Harold Simmons, a longtime Republican donor, gave $1 million. Democrats also took the top two positions for individual donors as well.

For a complete look at these donors, see the super PAC donor file on the bulk download page of Sunlight’s campaign finance tracker. This analysis disregarded loans, refunds and contributions earmarked through hybrid super PACs to other destinations.